Methods and apparatus for processing molten materials, and more particularly, methods and apparatus for conveying and/or atomizing molten materials using a nozzle are disclosed herein.
Critical powder metal components, such as turbine rotor disks, that are manufactured from nickel-base alloy powders must be manufactured using specialized processing and handling techniques to assure that the components are free from extremely small defects. This is because defects on the order of a few square thousandths of an inch can cause catastrophic failure of the components. As discussed below, one source of such defects in components manufactured from powders of nickel-base alloys is the ceramic nozzle commonly employed during manufacture of the powders to control the size of the molten metal stream and to direct it into the atomizing field.
More specifically, during atomization, molten metal is flowed from a vessel (for example a melting or refining furnace) through a nozzle to create a steam. On exiting the nozzle, the stream of molten metal is impinged with a fluid stream, which may be a liquid or a gas stream, to break-up or atomize the molten metal into droplets. The molten metal droplets cool to form powders as they fall from the atomization zone into a collection chamber. Because of the very high temperatures required to melt these superalloys, ceramic or refractory-lined nozzles have been used in the atomization process. One example of a ceramic nozzle is disclosed in British Patent No. GB 2154901 A and one example of a refractory-lined nozzle is disclosed in U.S. Pat. No. 1,545,253.
However, while ceramic and refractory-lined nozzles are advantageous in that they can withstand high processing temperatures, it has been found that the reactivity of many molten metals (such as nickel-base or titanium-base alloys) and the rapid flow of molten metal through the nozzle can cause erosion or degradation of the ceramic or refractory-lining. As the ceramic erodes, particles (i.e., erosion debris) are entrained in the molten metal stream. If the particles are too large to pass through the nozzle, the nozzle will become clogged, thereby stopping production. On the other hand, if the particles are small enough to pass through the nozzle, the particles will be incorporated into the metal powders or will be collected with the metal powders in the collection chamber. The presence of these particles in the atomized metal powder, either as inclusions in the metal powder or as separate particulate matter, is deleterious to the quality of the metal powders. For example, because ceramic inclusions can act as stress-concentrations sites, metal components formed from powders containing ceramic particles (either as inclusions in the powder or as separate particulate matter) can fail prematurely. Although it is possible to remove ceramic particles larger than some critical size by screening, this both increases the cost of the powders and creates scrap.
One alternative to ceramic nozzles that has been investigated is water-cooled copper nozzles having an induction heating coil positioned around the perimeter of the nozzle to inductively heat the molten metal flowing through the nozzle. One example of such a nozzle is disclosed in U.S. Pat. No. 5,272,718. However, because copper has a melting temperature significantly lower than the melting temperature of the alloys being processed, the copper nozzle itself cannot be heated to a high enough temperature to prevent solidification of the molten metal in the nozzle. Instead, the molten metal flowing through the nozzle must be inductively heated to prevent solidification. Further, the copper nozzle must be water-cooled to prevent the nozzle from melting or deforming during processing, and to allow a layer of solidified metal to form on the surface of the nozzle to prevent copper from the nozzle from dissolving in the molten metal. Since water-cooled, copper nozzles generally require frequent replacement and high power for operation, they can be costly to operate. Moreover, freeze-up of the nozzles due to solidification of molten metal either in the nozzle passageway or at the point of egress of the molten metal from the nozzle can be a frequent cause of process downtime.
Accordingly, there is a need for a nozzle that is compatible for use with high-temperature molten metals, such as nickel-base or titanium-base alloys. More particularly, there is a need for a nozzle that can withstand the high temperatures and environmental conditions associated with the atomization of nickel-base or titanium-base alloys, that can be directly heated to prevent freeze-up during processing, that can be readily monitored such that if the nozzle does fail the process can be stopped prior to forming a substantial quantity of metal powder that must be scrapped, and that can be rapidly cooled to permit the process to be quickly stopped if necessary or desired.